Fatty Acids in Bovine Milk Fat

Report
æ
Fatty acids in bovine milk fat
Helena Lindmark Månsson
Swedish Dairy Association, Lund, Sweden
Abstract
Milk fat contains approximately 400 different fatty acid, which make it the most complex of all natural fats.
The milk fatty acids are derived almost equally from two sources, the feed and the microbial activity in the
rumen of the cow and the lipids in bovine milk are mainly present in globules as an oil-in-water emulsion.
Almost 70% of the fat in Swedish milk is saturated of which around 11% comprises short-chain fatty acids,
almost half of which is butyric acid. Approximately 25% of the fatty acids in milk are mono-unsaturated and
2.3% are poly-unsaturated with omega-6/omega-3 ratio around 2.3. Approximately 2.7% are trans fatty acids.
Keywords: milk fat; fatty acid composition; bovine
Milk composition
Today, not only the nutritional value of milk but also
other physiological properties of milk components have
attracted interest (1). Bovine milk contains approximately
87% water, 4.6% lactose, 3.4% protein, 4.2% fat, 0.8%
minerals and 0.1% vitamins (2). The composition of milk
continuously undergoes changes depending on e.g. breeding, feeding strategies, management of the cow, lactation
stage and season (24).
Milk fat
The lipids in bovine milk are mainly present in globules
as an oil-in-water emulsion (5). These fat droplets are
formed by the endoplasmic reticulum in the epithelial
cells in the alveoli and coated with a surface material of
proteins and polar lipids. When secreted, they are
enveloped with the plasma membrane of the cell.
Membrane-associated materials can comprise 26% of
the globule mass (6, 7). The composition and structure of
the milk fat globule membrane (MFGM) is not known in
detail but it is mainly composed of polar lipids and
membrane-bound and associated proteins. The lipid
fraction comprising approximately 30% of the membrane
material consists of lipids such as phospholipids (25%),
cerebrosides (3%) and cholesterol (2%). The remaining
70% of the membrane material are proteins, many of
them being enzymes (8). The milk fat consists mainly of
triglycerides, approximately 98%, while other milk lipids
are diacylglycerol (about 2% of the lipid fraction),
cholesterol (less than 0.5%), phospholipids (about 1%)
and free fatty acids (FFA) (about 0.1) (9). In addition,
there are trace amounts of ether lipids, hydrocarbons, fatsoluble vitamins, flavour compounds and compounds
introduced by the feed (10). The size of the milk fat
globule (MFG) increases with increasing fat content in
the milk probably because of a limitation in production
of MFGM (11). The number of MFG in milk is
approximately 1010 per mL with a total area of 700 cm2
per mL of milk (4). The size of the MFG has crucial
influence on the stability and technological properties of
milk. Milk lipid globules are resistant to pancreatic
lipolysis in the small intestine unless they are first
exposed to gastric lipolysis (12).
Origin of milk fatty acids
The milk fatty acids are derived almost equally from two
sources, the feed and the microbial activity in the rumen
of the cow (10). The fatty acid synthesising system in the
mammary gland of the cow produces fatty acids with
even number of carbons of 416 carbons in length and
accounts for approximately 60 and 45% of the fatty acids
on a molar and weight basis, respectively (13). This de
novo synthesis in the mammary gland is of the 4:014:0
acids together with about half of the 16:0 from acetate
and b-hydroxybutyrate. Acetate and butyric acid are
generated in the rumen by fermentation of feed components. The butyric acid is converted to b-hydroxybutyrate
during absorption through the rumen epithelium. Bovine
fat contains certain fatty acids with odd number of
carbons, such as pentadecanoic acid (15:0) and heptadecanoic acid (17:0). These two fatty acids are synthesised
by the bacterial flora in the rumen (14). The remaining
16:0 and the long-chain fatty acids originate from dietary
lipids and from lipolysis of adipose tissue triacylglycerols
(10). Medium- and long-chain fatty acids, but mainly
18:0, may be desaturated in the mammary gland to form
the corresponding monosaturated acids.
Fatty acids are not randomly esterified at the three
positions of the triacylglycerol molecule (5). The shortchain acids butyric (4:0) and caproic (6:0) are esterified
Food & Nutrition Research 2008. # 2008 Lindmark Månsson H. This is an Open Access article distributed under the terms of the Creative Commons AttributionNoncommercial 3.0 Unported License (http://creativecommons.org/licenses/by-nc/3.0/), permiting all non-commercial use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Citation: Food & Nutrition Research 2008. DOI: 10.3402/fnr.v52i0.1821
1
(page number not for citation purpose)
Månsson H. L.
almost entirely at sn-3. Medium-chain fatty acids (8:0
14:0) as well as 16:0 are preferentially esterified at
positions sn-1 and sn-2. Stearic acid (18:0) is selectively
placed at position sn-1, whereas oleic acid (18:1) shows
preference for positions sn-1 and sn-3.
When consumed by humans, milk triacylglycerols are
lipolysed by lingual lipases in the mouth and by both
lingual and gastric lipase in the stomach (10). The lipases
preferentially hydrolyse sn-3 position fatty acids, and
therefore selectively releases the shorter acids. The result
is that 4:010:0 pass through the stomach wall in
decreasing quantities as the molecular weight increases,
enter the portal vein, and are transported to the liver
where they are oxidised. About 2540% of the triacylglycerols are digested in the stomach (15).
Fatty acid composition
Milk fat triacylglycerols are synthesised from more than
400 different fatty acids, which makes milk fat the most
complex of all natural fats (9, 10, 15). Nearly all of these
acids are present in trace quantities and only about 15
acids at the 1% level or higher. Many factors are
associated with the variations in the amount and fatty
acid composition of bovine milk lipids (15, 16). They may
be of animal origin, i.e. related to genetics (breed and
selection), stage of lactation, mastitis and ruminal
fermentation, or they may be feed-related factors, i.e.
related to fibre and energy intake, dietary fats, and
seasonal and regional effects. The gross composition of
milk fat in Swedish dairy milk 2001 was 69.4% saturated
fatty acids and 30.6% unsaturated fatty acids (LindmarkMånsson, 2001). The content of saturated fatty acids is
lowest in the summer when the cows are grazing, and
highest in the winter due to indoor feeding. The content
of the unsaturated fatty acids shows the opposite pattern
with the highest amount in the summer. The fatty acid
composition of Swedish milk fat is given in Table 1.
The saturated fatty acids present in milk accounts for
approximately 70% by weight (5). The most important
fatty acid from a quantitative viewpoint is palmitic acid
(16:0), which accounts for approximately 30% by weight
of the total fatty acids. Myristic acid (14:0) and stearic
acid (18:0) make up 11 and 12% by weight, respectively.
Of the saturated fatty acids, 10.9% are short-chain fatty
acids (C4:0C10:0). The amounts of butyric acid (4:0)
and caproic acid (6:0) on a yearly average are 4.4 and
2.4% by weight of the total fatty acids, respectively, in
Swedish dairy milk (2). These amounts are higher when
their proportions are expressed as molar percentages,
approximately 10 and 5%, respectively (5).
Approximately 25% of the fatty acids in milk are
mono-unsaturated with oleic acid (18:1) accounting for
23.8% by weight of the total fatty acids in Swedish
dairy milk. Poly-unsaturated fatty acids constitute
2
(page number not for citation purpose)
Table 1. Fatty acid composition expressed as percent by weight of
total fatty acids in Swedish dairy milk in 2001, given as weighted
means with standard deviations (SD) and as the minimum and maximum weighted means. The estimation of the weighted mean values
was based on the proportion of milk delivered to each dairy or dairy
company at each sampling occasion (seven dairies at four sampling
occasions during 2001). The lowest and highest values observed and
p-values for geographical and seasonal variation are also given
Lowest
Weighted
Fatty acid
value
mean 2001 SD observed
Highest
value
Seasonal
observed
variation
4:0
4.4
0.1
4.0
5.1
n.s.
6:0
2.4
0.1
2.1
2.9
n.s.
8:0
1.4
0.1
1.2
1.9
n.s.
10:0
12:0
2.7
3.3
0.2
0.2
2.4
3.0
3.5
4.1
*
**
14:0
10.9
0.5
10.0
12.1
***
15:0
0.9
0.0
0.8
1.1
n.s.
16:0
30.6
0.9
28.7
34.1
**
17:0
0.4
0.0
0.4
0.5
**
18:0
12.2
0.4
10.3
13.3
n.s.
20:0
0.2
0.0
0.2
0.2
Saturated fatty
acids total
10:1
14:1
16:1
17:1
18:1
Mono-unsaturated faty acids,
cis, total
18:2
18:3
Poly-unsaturated
fatty acids, cis,
total
16:1t
18:1t
18:2t
Trans fatty acids
total
CLA
69.4
1.7
67.1
74.4
0.3
0.0
0.2
0.4
n.s.
0.8
0.4
0.4
1.3
**
1.0
0.0
0.9
1.8
n.s.
0.1
0.0
B0.1
0.3
22.8
1.0
19.7
24.7
***
25.0
1.0
22.2
26.7
**
1.6
0.1
1.4
1.8
n.s.
0.7
0.0
0.6
0.9
**
2.3
0.1
2.0
2.5
n.s.
0.4
0.1
0.3
0.4
***
2.1
0.2
0.7
0.0
2.0
0.1
3.3
0.5
***
n.s.
2.7
0.7
0.6
3.9
***
0.4
0.1
0.3
0.5
***
n.s.
***
n.s.
n.s.: Not significant; *p B0.05; **p B0.01; ***p B0.001.
about 2.3% by weight of the total fatty acids and the
main poly-unsaturated fatty acids are linoleic acid (18:2)
and a-linolenic acid (18:3) accounting for 1.6 and 0.7%
by weight of the total fatty acids. The ratio between
omega-6 and omega-3 fatty acids in Swedish milk fat was
2.3:1 in 2001 (2). Milk and meat from ruminants can be
Fatty acids in bovine milk fat
an important source of omega-3 fatty acids in the human
diet, as is the case in France, where animal products
account for about 40% of the intake (17).
Approximately 2.7% of the fatty acids in milk are trans
fatty acids with one or more trans-double bonds (18). The
main trans 18:1 isomer is vaccenic acid (VA), (18:1, 11t),
but trans double bounds in position 416 is also observed
in low concentrations in milk fat (5). VA constitutes
approximately 2.7% of the total fatty acid content and
varies with season. Milk fat contains also conjugated
linoleic acid (CLA), with many different isomers including rumenic acid (RA) (cis-9, trans-11 CLA) which
predominates (7590% of total CLA) (18). The majority
of RA in the milk fat is synthesised endogenously, in
the mammary gland through the action of mammary
D-desaturase on VA (19). Thus both VA and RA are
present in milk and dairy products, generally in the ration
of about 1:3 (20). VA has a double role in metabolism
because it is both a trans fatty acid and a precursor for 9c,
11t-CLA. A small amount of the CLA originates from
biohydrogenation of unsaturated fatty acids by rumen
bacteria. Animal studies and new human data have
confirmed the bioconversion of VA into CLA (21, 22).
Bovine milk, milk products and bovine meat are the main
dietary sources of the RA (23). Milk content of 9c, 11tCLA varies considerably, but in Swedish milk fat it
constitutes about 0.4% of the fat fraction.
References
1. Miller GD, Jarvis JK, McBean LD. Handbook of dairy foods
and nutrition. Boca Raton, FL: National Dairy Council; 2007.
2. Lindmark Månsson H. Composition of Swedish dairy milk
2001. Report Nr 7025-P (In Swedish), Swedish Dairy Association; 2003.
3. Walstra P, Jenness R. Dairy chemistry and physics. New York:
John Wiley & Sons; 1984.
4. Walstra P, Geurts TJ, Noomen A, Jellema A, van Boekel MAJS.
Dairy technology: principles of milk properties and processes.
New York: Marcel Dekker, Inc; 1999.
5. MacGibbon AHK, Taylor MW. Composition and structure of
bovine milk lipids. In: Fox PF, McSweeney PLH, eds. Advanced
dairy chemistry. New York: Springer; 2006. p. 142.
6. Evers JM. The milk fat globule membrane-composition and
structural changes post secretion by the mammary secretory
cell. Int Dairy J 2004; 14: 66174.
7. Keenan TW, Mather IH. Milk fat globule membrane. In:
Roginski H, Fuquay JW, Fox PF, eds. Encyclopedia of dairy
sciences. London: Academic Press; 2003. p. 156876.
8. Mather IH. A review and proposed nomenclature for major
proteins of the milk-fat globule membrane. J Dairy Sci 2000; 83:
20347.
9. Jensen RG, Newburg DS. Bovine milk lipids. In: Jensen RG, ed.
Handbook of milk composition. London, UK: Academic Press;
1995: 54375.
10. Parodi P. Milk fat in human nutrition. Australian J Dairy
Technol 2004; 59: 359.
11. Wiking L, Stagsted J, Bjorck L, Nielsen JH. Milk fat globule
size is affected by fat production in dairy cows. Int Dairy J 2004;
14: 90913.
12. Noble RC. Digestion, absorption and transport of lipids in
ruminant animals. Prog Lipid Res 1978; 17: 5591.
13. McGuire MA, Bauman DE. Milk biosynthesis and secretion.
In: Roginsky H, Fuquay JW, Fox PF, eds. Encyclopedia of dairy
science. New York: Academic Press; 2003. p. 182834.
14. German JB, Dillard CJ. Composition, structure and absorption
of milk lipids: a source of energy, fat-soluble nutrients and
bioactive molecules. Crit Rev Food Sci Nutr 2006; 46: 5792.
15. Jensen RG. The Composition of Bovine Milk Lipids: January
1995 to December 2000. J Dairy Sci 2002; 85: 295350.
16. Palmquist DL, Beaulieu AD, Barbano DM. Feed and animal
factors influencing milk fat composition. J Dairy Sci 1993; 76:
175371.
17. French Food Safety Agency. The omega 3 fatty acids and the
cardiovascular system; nutritional benefits and claims; 2002.
http://www.afssa.fr/Documents/NUT-Ra-omega3EN.pdf ed
18. Precht D, Molkentin J. Trans fatty acids: implications for health,
analytical methods, incidence in edible fats and intake (a
review). Nahrung 1995; 39: 34374.
19. Bauman DE, Lock AL. Conjugated linoleic acid: biosynthesis
and nutritional significance. In: Fox PF, McSweeney PLH, eds.
Advanced dairy chemistry. New York: Kluwer Academic/
Plenum Publishers; 2006. p. 93136.
20. Lock AL, Bauman DE. Modifying milk fat composition of
dairy cows to enhance fatty acids beneficial to human health.
Lipids Health Dis 2004; 39: 1197206.
21. Mosley EE, McGuire MK, Williams JE, McGuire MA. Cis-9,
Trans-11 conjugated linoleic acid is synthesized from vaccenic
acid in lactating women. J Nutr 2006; 136: 2297301.
22. Turpeinen AM, Mutanen M, Aro A, Salminen I, Basu S,
Palmquist DL, et al. Bioconversion of vaccenic acid to
conjugated linoleic acid in humans. Am J Clin Nutr 2002; 76:
50410.
23. Wahle KW, Heys SD, Rotondo D. Conjugated linoleic acids: are
they beneficial or detrimental to health? Prog Lipid Res 2004;
43: 55387.
Helena Lindmark Månsson
Swedish Dairy Association, Ideon Science Park,
SE-223 70, Lund, Sweden
E-mail: [email protected]
3
(page number not for citation purpose)